Different methods exist for fabricating semiconductor substrates. These substrates are generally in the form of wafers that may be disc-shaped (e.g., with a diameter of 200 or 300 mm), or a different shape.
The choice of fabrication method generally depends on the type of semiconductor material, on the intended application and on economic constraints.
For example, to obtain silicon substrates intended for the fabrication of photovoltaic cells, it is possible to use a re-solidification method of molten silicon to obtain so-called polycrystalline or multi-crystalline silicon having coarse particles with a preferred direction of orientation. With this method, it is not possible to obtain substrates in single-crystalline material, although it is generally desired to have single-crystalline material for microelectronic applications.
Additionally, this method is not adapted for obtaining a substrate from a donor substrate in which a weakened zone has been fainted.
It may effectively be desired, for some applications, to form a substrate in semiconductor material of controlled thickness and quality, and comprising a weakened zone in the thickness of the substrate at a depth that is also controlled.
In this text, by “weakened zone” is meant a region that extends underneath a main face of the substrate (for a disc, underneath one of the faces of the disc) and parallel to this face, at a constant depth in the thickness of the substrate and that, under the action heating optionally combined with mechanical stress, will lead to the detachment of the substrate at this region.
The weakened zone may typically be formed by inserting ionic species, e.g., hydrogen and/or helium, and/or rare gases into the substrate, to generate microcavities therein forming the weakened zone. This inserting can be achieved by implantation or even by other means such as species diffusion. One known example using the insertion of species to create a weakened zone is the SMART CUT® process. The microcavities, under the effect of a heat budget, can then set up stresses in the substrate that will lead to the detachment, alone or in combination with mechanical stresses applied to either side of the weakened zone.
One advantageous application of the invention is the fabrication of substrates comprising a weakened zone, allowing the detachment of a layer of semiconductor material. In this manner, the invention allows the forming of layers whose thickness can be controlled with great precision, even at low thickness values, e.g., of the order of 50 microns. With the invention, it is also possible to fabricate layers of semiconductor material, still of controlled thickness and possibly of very narrow thickness, in single-crystalline material of very good quality.
To prepare silicon substrates for application in microelectronics, it is known to use the Czochralsky pulling process, which allows the fabrication of ingots of single-crystal silicon.
Ingot pulling using the Czochralsky process may give rise to problems, in particular if the ingot is of large diameter (e.g., 300 mm or 450 mm). When it is desired to increase the diameter of the ingot and hence of the substrates derived therefrom, the extraction of the latent heat from solidification of the ingot becomes increasingly more difficult. This limits ingot pulling rates. In addition, it is likely to create non-homogeneities inside the ingot.
The two above-mentioned methods yield silicon ingots that must then be cut by sawing to obtain substrates. It is also generally necessary to treat the surfaces of these substrates.
These cutting and treating operations are relatively costly.
In addition, they have the disadvantage of consuming a significant part of the ingot mass, and they can only lead with difficulty—even not at all—to the obtaining of thin substrates.
For example, by sawing an ingot, it is not possible to obtain thin substrates having a thickness of 50 microns, for example. Yet, for some applications, such as the manufacture of photovoltaic cells, a narrow substrate thickness may be desired.
Obtaining layers of single-crystal material by epitaxial growth on a single-crystal starting substrate by means of a vacuum deposit method, for example, of CVD type, or using a molecular beam epitaxy method (MBE) is also known. Under well-controlled conditions, a method for fabricating layers by epitaxial growth can produce material of good quality. Limitations of this type of method follow from the slowness of the process and its cost.